A transmission valve has to have a high level of robustness in order to be fail safe when transmission oil change intervals are very long, in the extreme when there are so called service life fillings. A high level of robustness can be achieved when moving parts have large clearances. However, this degrades control quality.
A hydraulic valve is known from DE 10 2011 053 023 A1 which has a high level of robustness in addition to high control quality. The high level of robustness is achieved in that contaminant particles in the operating medium cannot cause the transmission valve to jam since the armature can impart axial forces that are high enough so that the armature can always pull itself clear. At the same time the hydraulic valve provides a high level of control quality which is achieved by several design measures. Thus, in particular transversal forces between the armature and a pole tube are minimized.
A design measure of this type for minimizing transversal forces is a very tight operating clearance between the armature and the pole tube which is achieved with a very thin separation layer instead of for example a non-magnetic sleeve or a non-magnetic coating. A very thin separation layer of this type is advantageously provided with a layer thickness of less than 60 μm. The thin separation layer can be applied for example chemically or through a galvanic process. For a chemical method, for example chemical nickel plating can be used.
In order to obtain a proportional behavior of the transmission valve, DE 10 2011 053 023A1 provides a pole core cone. The pole core cone of this type also facilitates implementing other force/distance diagrams. However, a linear force/distance diagram is typically desirable to simplify regulation.